Please avoid using SAC headers after the initial mtuq.read() call

[rmodrak]

To allow support for other file formats beyond SAC, please try to adopt the following

• the initial mtuq.read() call handles all file format dependent metadata parsing
• after that, metadata are stored in a common internal structure that works the same regardless of the original file format
• in particular, this means not using observed data SAC headers at any point after the initial mtuq.read() call
• the whole motivation is that data processing and inversion functions should work the same, regardless of the original file format